The invention is directed to a method for the ultrasonic measurement of the running time and quantity of a flowing fluid, in which the running time of an ultrasonic signal is measured counter to and in the direction of the flow, wherefrom the flow rate of the fluid is determined. In addition, the invention concerns a device for carrying out such a method.
The ultrasonic running time method is a known and proven method for the determination of the flow rate of fluids such as fluid or gaseous media in pipes or ducts. For an ultrasonic running time method according to the state of the art (FIG. 1), at least two ultrasonic transducers 12a, 12b are arranged staggered towards the direction of flow R in a flow-through measurement cross section 10. The distance 1 between the two ultrasonic transducers 12a, 12b is greater than the width b of the flow-through measurement cross section. In this respect, the ultrasonic transducers 12a, 12b can be both wetted by a medium as well as attached from the outside to the wall of the measurement cross section 10. By means of the inherent speed of the medium, ultrasonic signals that are sent diagonally with the flow downstream require a lower running time t1 than the ultrasonic signal that is sent upstream (running time t2). If the distance between the ultrasonic transducers as well as the angle θ between ultrasonic path and direction of flow are known, then the average flow rate can be determined from the difference in running times dt=t2−t1:
  v  =            l              2        ⁢        cos        ⁢                                  ⁢        θ              ·          dt                        t          1                ·                  t          2                    
To determine the difference in running times, in modern measurement devices the received signals are digitized and then analyzed with an analysis computer (Digital Signal Processor, DSP). Multiple-path ultrasonic flow-through measurements are used because of the complex flow conditions for flow-through measurement in large cross-sections and for flow-through measurement for free-flow cross-sections. As an example, FIG. 2 shows one such flow-through measurement in a measurement cross-section 10 in the form of an open channel in which a fluid 20 with a liquid level hp flows. In this example, horizontal measurement paths 1 through n are arranged on top of each other between ultrasonic transducers 12a, 12b, 14a, 14b, 16a, 16b, 18a, 18b arranged in pairs such that the unequal speed distribution v1 through vn over the height of the liquid levels h1 through hn (FIG. 3) can be determined. By means of a suitable integration, the average flow rate in the cross-section can thus be calculated with high accuracy. Such measurement methods are described in the ISO/DIS 6416, for example.
For many applications for which the ultrasonic flow-through analyzer comes into use, the measurement of the particle density or diffusion would be of great interest alongside the measurement of the amount of flow-through. As examples are mentioned here the measurement of flue- and exhaust gases, the measurement of air-flow streams for combustion control, the measurement behind filters and the measurement of inflows and outflows in waste water treatment plants. In the mentioned examples, the measurement of particles is partially specified in order to fulfill environmental requirements, for example, and/or it is an important parameter in order to optimally monitor or control the processes.
There are processes for the measurement of the concentration of particles according to known optical principles. In doing so the attenuation of a light by means of scattering and absorption is used for the calculation of the concentration of the particles in the fluid. However, these methods monitor a very small control volume that is assumed to be representative for the entire measurement cross-section. In addition, these methods provide no locally-resolved measurement value. A further disadvantage is that these methods are susceptible to contamination and thus maintenance-intensive, for example, against the buildup of algae or the deposit of suspended matter on the light sources or sensors.